Following points: 

• testing
• testing
• testing 

Note that we xxxx 

1. Abbreviations

• SDK - Software Development Kit
• HAL - Hardware Abstraction Layer
• KBA - Knowledge Base Article
• ROS - Robot Operating System
• SLSTK, SLSTK3701A - Giant Gecko Starter Kit

2. Configuration

• Board:
  • EFM32GG11B STK - BRD2204A Rev BD4 (SLSTK3701A)
• Development:
  • Windows 10 OS
  • Simplicity Studio V4.
  • 32-bit MCU SDK Version: 5.6.1.0
• Execution:
  • Ubuntu 16.04 LTS
  • ROS Kinetic

3. Preface

The ROS is one of the most known robot platform which hobbyist uses for their robot projects. Furthermore, based on the ROS related widespread libraries, the user community, and the well-structured documentation, usage of the ROS expanded to the industrial participants too. ROS' name refers to an operating system, but it is not truly an operating system, instead, ROS is a framework which should be installed and configured to run on Linux operating system. The official ROS releases recommend that the developers should install these frameworks to one of Linux Ubuntu distros. As regards of the resource consumption of Ubuntu distributions, the ROS cannot be run on low resource based MCUs (like Cortex M, Atmel 8bit, etc.), so the developer should use appropriate devices to run Ubuntu. These devices can be regular PC's, laptops or board computers (e.g.: Raspberry Pi). At this point the reader probably has a question: why is it useful using of low power MCUs when the ROS cannot be run on them. And the answer is: these MCUs are used as expansions of the ROS’ host computer. An MCU can easily initiate a connection with a sensor, collect the data of this sensor and at the end send the collected data to the host PC where the data will be processed. The officially supported MCUs are listed on the ROS website, and it is also described how a not supported one can be added. This KBA will give a high-level description with an example about how EFM32 or EFR32 MCUs can be transformed to a ROS Serial Node.

4. What can the reader expect from this KBA

This KBA will not explain: ROS related base concepts. The readers who are not familiar with those words in ROS environment like roscore, node, topic, etc. should take the first steps on the ROS website. The intent of this KBA is describing the process of how a developer can append a not officially ROS supported MCU to ROS environment. The description will not start from the scratch: the ROS related chapter about how a new MCU can be added, the Arduino related source codes and the Silabs related SDK will be used for the development. The previously used "development" word can be divided to two parts: in the first part the ROS related hardware driver will be implemented, in the second part an available Arduino based ROS end node project will be ported to the previously implemented end node.

5. ROS serial node

The serial node's operation is very primitive: the communication between the host PC and MCU is performed via UART. This communication is defined by ROS, the ROS library has module which defines it. Before the MCU starts to exchange data between itself and the host PC, the MCU has to convert the data to a predefined structure, which called "messages". These messages also have modules in the ROS library, so they can be just used. The following picture will visualize the high-level structure of the implemented serial node.

5.1 ROS library - HW linking layer

The description starts with the middle layer, because it is the key element contributing to the operation of the node. Check the following site for initial description: (http://wiki.ros.org/rosserial_client/Tutorials/Adding%20Support%20for%20New%20Hardware) All lower layers, software and hardware, shall be encapsulated into the following 4 methods, and upper ROS layers, like ROS library and ROS application will use these methods:

class Hardware
{

  Hardware();

  // any initialization code necessary to use the serial port
  void init(); 

  // read a byte from the serial port. -1 = failure
  int read()

  // write data to the connection to ROS
  void write(uint8_t* data, int length);

  // returns milliseconds since start of program
  unsigned long time();

};

The reader can find the SLWSTK3701 related class file in the attached project, with the following path and name: /ros_lib/ros/SLSTK3701AHardware.h. Another specialty of this layer is that it is a mixture of the c and the c++ programming language. The HAL layer has been written in C, but the ROS related layers require C++ methods and classes.

Explanation of the functions:

void init():

This method is responsible for the initialization of the hardware, the MCU and the MCU related peripherals. It has 2 mandatory items which shall be initialized:

• a TIMER peripheral, which will be the base of the time() method and
• an USART peripheral, which will be the base of read() and write() methods.

int read():

Read() method is responsible for receiving the data which has been sent by the host PC. The ROS Lib processes the received data in asynchronous mode, therefore the plain USART_Rx() function had to be expanded with a circular buffer. This buffer was not part of the ROS Lib, so it has been implemented, too. The circular buffer has been placed under the \utils\ringbuffer_class folder.

void write():

Write() method is the opposite of the read() regarding the functionality. Its only one role is place data to the used USART line. The host side (PC side) can receive this data without any additional buffering method, providing the host side implements it.

unsigned long time():

Return value of this method shall be incremented with one after each elapsed millisecond.

The following figure highlights where the Linking Layer is placed in the Simplicity Studio project structure.

Figure 2 - Linking layer location

5.2 ROS library

The ROS library has been downloaded from the ROS website, which marked as Arduino library. The necessary modules for the operation of the example application are attached for the example project. These files can be found under ros_lib/ path.

Figure 3 - ROS Lib location

5.3 Application

In this chapter, we will examine the operation of the base application (which is one of the original Arduino ROS examples), the ported application and at the end, we will compare them. The chosen application is one of the simplest Arduino related ROS_lib example which name is: blink. The expected operation of the original Arduino example is:

• build the application and upload it to an Arduino Uno
• connect the Arduino to the host computer of ROS
• enter to a terminal: roscore
• enter to another terminal: rosrun rosserial_python serial_node.py /dev/ttyUSBn
• enter to a 3rd terminal: rostopic pub toggle_led std_msgs/Empty –once

The last task can be done more times. Each repetition will cause blinking of the Arduino's LED.

The following figure compares the codes used for of Arduino and the one used for of Simplicity Studio.

Figure 4 - Comparasion of the 2 source codes

In case of SLSTK3701A, the operation will be the same as the original one, but there are differences with the setup. In steps:

• the first thing is: compile the attached project and upload the compiled *.hex file to the SLSTK
• then connect the SLSTK to the host computer, according to the following description: the ROS node initialized as to use the USART0 peripheral, which means ROS node uses the extension header related pins. The pin 4 works as Rx and the pin 6 is the Tx. I used one of the Silabs developed USART - USB converters to establish the connection between the SLSTK and the host PC, but any other appropriate device can be used.
• after the previous point the steps to take for Arduino and STK are same, so enter to a terminal: roscore
• enter to another terminal: rosrun rosserial_python serial_node.py /dev/ttyUSBn
• enter to a 3rd terminal: rostopic pub toggle_led std_msgs/Empty –once

5.4 HAL (Peripheral drivers)

It is Silabs SDK related peripheral drivers. The following figure highlights the used HAL elements for the project.

Figure 5 - HAL location

5.5 HW - EFM32GG11B_SLSTK3701A

The used development board is one of the Silicon Lab's flagship models. The ability of the EFM32GG11B is much higher than the used application requirements, but this MCU and development board is the most known board, therefore this one used for this demo project.